Driving devices of this type, which can be rotary or linear devices, make it possible, in particular, to provide along a relatively long path of the movable member a torque or a force which is constant for a given value of the ampere-turns of the energizing coil, this torque or this force being proportional to the ampere-turns. Such devices can thus be used advantageously as directly controlling actuators, which are more reliable than those formed by d.c. motors with reduction gears. They can also be used for example as polyphase rotary motors providing, when fed with constant current, a trapezoidal torque characteristic in each phase.
An inconvenience of these known devices resides in the cost of the high energy magnetic materials, such as generally necessary for achieving the required performances, in particular regarding the force or the torque to be delivered.
Another inconvenience of the known devices is the fact that in order to reach sufficient forces or torques, the movable member is provided with elementary permanent magnets of alternating polarities and therefore requires a magnetization prior to the assembly of the device and manipulations which are critical in view of the energy and the fragility of the magnets being used. Therefore, and in particular in large series manufacturing, the risk cannot be entirely excluded that a magnet particle remains inside the device and lodges in the air-gap of a magnetic circuit, thereby causing a defect of the device.